Sacrificial polymer layers or compositions that encompass a sacrificial polymer, such as, polypropylene carbonate (PPC), and a photoacid generator are known. Typically such sacrificial polymer layers are formed overlying a substrate, by applying a sacrificial polymer composition, encompassing a sacrificial polymer and a photoacid generator (PAG), to the substrate. The resulting sacrificial polymer layer, having a uniform distribution of PAG therethrough, is then photo-patterned by first performing an imagewise exposure to actinic radiation and then removing exposed portions of the polymer layer by the thermal decomposition of such portions. This patterning is then generally followed by the forming of an overcoat layer that essentially encapsulates the remaining portions of the sacrificial polymer layer. Typically such overcoat layer is a non-sacrificial material. The overcoated, photo-patterned sacrificial polymer structure is then heated to a temperature sufficient to cause the encapsulated sacrificial polymer to decompose and further to cause decomposition products formed to pass or permeate through the overcoat layer. In this manner, a defined three dimensional enclosed space, i.e., an air cavity or air gap, is formed. While such known processes have proven effective for some applications, it has been found that (1) after decomposition, the amount of residue remaining in the enclosed space, such as from the PAG additive, could be problematic for the creation of advanced devices and/or the use of such advanced devices in both new and existing applications; and (2) that patterning such layers often results in structures having a high degree of line edge roughness (LER).
For example, FIG. 1 shows a top view of residue observed after the decomposition of an unencapsulated sacrificial polymer structure made in accordance with the prior art as described above. That is to say, that such structure was not overcoated prior to heating to cause decomposition. The average thickness of the residue seen was measured by scanning profilometry and calculated to be about 0.157 μm. Based on the original thickness of the structure, the thickness of the residue represents about 0.55% residue.
Referring now to FIG. 2, which shows a top view of a photo-patterned sacrificial polymer structure, prior to encapsulation, made in accordance with the prior art. As seen, there is significant non-uniformity of the width for the vertical lines which is often referred to as line edge roughness (LER). Additionally, significant variation in the thickness of the dark peripheral regions can be observed, which are due to diffusion of the acid created by activation of the PAG into the unexposed region. One of skill in the art will recognize these variations as an indication of variations in the slope of the sidewalls of the feature shown; thicker dark regions being indicative of a more sloped sidewall.
Further, as presently known sacrificial polymer layers having the aforementioned uniform distribution of PAG therethrough encompass high total amounts of PAG additive, such layers are generally highly absorbing of the actinic radiation required for PAG activation. Thus the high radiation doses needed can also be problematic as they lead to long exposure times which can adversely affect manufacturing throughput.
Thus, as the amount of residue, degree of LER observed and long exposure times needed for PAG activation for known sacrificial polymer layers made by known methods can be problematic, it would be desirable to develop sacrificial polymer layers and methods for forming and decomposing such layers that address the aforementioned deficiencies